1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to servo writing a disk drive by writing spiral tracks using a mechanical position sensor.
2. Description of the Prior Art
FIG. 1 shows an exploded view of a prior art disk drive comprising a disk 2 rotated by a spindle motor 4, and a head 6 coupled to a distal end of an actuator arm 8 which is rotated about a pivot 10 by a voice coil motor (VCM) in order to actuate the head 6 over the disk 2. The disk 2, spindle motor 4, head 6, actuator arm 8, and VCM are enclosed in a head disk assembly (HDA) comprising a base 9 and a cover 11. The VCM comprises a voice coil 12 coupled to the base of the actuator arm 8 and one or more permanent magnets attached to a yoke 14. When the voice coil 12 is energized with current, the resulting magnetic flux interacts with the magnetic flux of the permanent magnets to generate a torque that rotates the actuator arm 8 about the pivot 10. A tang 16 attached to the actuator arm 8 interacts with a crash stop 18 to limit the stroke of the actuator arm 8, and also provides a latching mechanism (e.g., using a magnet) to maintain the actuator arm 8 in a latched position while the disk drive is powered down. Alternatively, the actuator arm 8 may be parked on a ramp located at the outer periphery of the disk 2 when the disk drive is powered down.
Servo sectors 210-21N are written to the disk 2 to define a plurality of radially spaced, concentric servo tracks 23 as shown in the prior art disk format of FIG. 2. A plurality of data tracks are defined relative to the servo tracks 23, wherein each data track is partitioned into a plurality of data sectors with embedded servo sectors 210-21N. Each servo sector (e.g., servo sector 214) comprises a preamble 25 for synchronizing gain control and timing recovery, a sync mark 27 for synchronizing to a data field 29 comprising coarse head positioning information such as a track number, and servo bursts 31 which provide fine head positioning information. The coarse head position information is processed to position a head over a target track during a seek operation, and the servo bursts 31 are processed to maintain the head over a centerline of the target track while writing or reading data during a tracking operation.
In the past, external servo writers have been used to write the product servo sectors 210-21N to the disk surface during manufacturing. External servo writers employ extremely accurate head positioning mechanics, such as a laser interferometer, to ensure the product servo sectors 210-21N are written at the proper radial location from the outer diameter of the disk to the inner diameter of the disk. However, external servo writers are expensive and require a clean room environment so that a head positioning pin can be inserted into the HDA without contaminating the disk. Thus, external servo writers have become an expensive bottleneck in the disk drive manufacturing process.
The prior art has suggested various “self-servo” writing methods wherein the internal electronics of the disk drive are used to write the product servo sectors independent of an external servo writer. In one prior art technique, a self-servo writing operation is performed by writing a plurality of spiral tracks to the disk which are then processed to write the product servo sectors along a circular path. The spiral tracks are written “open loop” by seeking the head from an outer diameter of the disk to an inner diameter of the disk. The disk drive calibrates acceleration/deceleration impulses to seek the head from the outer to inner diameter in a desired amount of time. Accurate radial positioning of the spiral tracks assumes the calibration process is accurate and that the calibrated acceleration/deceleration impulses will generate a repeatable response over multiple seeks. However, the calibration process will inevitably exhibit some degree of error, and the dynamics of the disk drive will change between seeks inducing errors in the radial position of the spiral tracks. Dynamic errors which degrade the spiral tracks written during an open loop seek include vibration of the HDA, flutter and non-repeatable run-out of the disk and spindle bearings, stiction and non-repeatable run-out of the pivot bearings, windage on the head and arm, and flex circuit bias, windage and vibration. Errors in writing the spiral tracks will propagate to the product servo sectors, thereby degrading the operating performance of the disk drive and reducing the manufacturing yield.
There is, therefore, a need to improve the servo writing process for a disk drive by reducing the bottleneck and expense of external servo writers while maintaining adequate operating performance and manufacturing yield.